menuconfig: factor out reserved letters

These letters that should not be used in menu selection.

Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
menuconfig: allow j/k to move down/up the menu

Like in Vim.

Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>